1. Field of the Invention
The present invention relates to a method of denitrification of water, and more particularly to a method that reduces the need for chemicals typically used in denitrification by relying primarily on gas (air) stripping for the denitrification.
2. Prior Art
Biological denitrification is the most common method used today. However, biological denitrification suffers from a number of important disadvantages. These include slow reaction times, and, typically, the requirement for the use of large equipment. Furthermore, it is difficult to keep a viable culture of the necessary bacteria, and expensive to purchase the chemicals required to maintain the bacterial culture. Moreover, the bacterial culture maintained often itself contaminates drinking water. Additionally, bacterial cultures are frequently supplied with an organic source of carbon, such as methanol, but often some methanol remains in the water and becomes an organic contaminant thereby making biological denitrification unacceptable in most drinking water applications. Biological denitrification is an unpredictable process because throughout any given year, the nitrate level in water supply can change, and so can the size of the bacterial culture. Further complicating the use of biological denitrification is the inability to turn the process on and off like a mechanical system.
Selective ion exchange processes, such as that disclosed in U.S. Pat. No. 4,479,877 to Guter are used for denitrification mostly in drinking water applications. However, there are numerous costs associated with the ion exchange resins used in the processes. Furthermore, there is a loss of ion exchange capacity due to the oxidation of ion exchange functionalities over time. The ion exchange resins are not as rugged as the cationic ones and the amines or quaternary ammonium groups oxidize and need replacement. There are additional expenses associated with the costs of regenerant solutions and the disposal costs of waste regenerant solutions. Lastly, selective ion exchange processes are not practical at high nitrate concentrations, such as those concentrations that typically occur in irrigation runoff waters.
A third method of denitrification uses biological denitrification and the recycling of waste regenerant solutions for ion exchange processes. This method combines the two methods mentioned above, but also suffers from similar disadvantages. Specifically, the nutrient costs associated with keeping the microorganisms alive become expensive. There are also contamination concerns resulting from the microorganisms themselves. Also, the organics (i.e., carbon sources) that are supplied for the bacteria can cause contamination if these organics remain in the drinking water. Furthermore, reaction times are slow and there are other problems resulting from the unpredictable nature of the changes in nitrate levels and the size of the bacterial culture. Lastly, the dilution of the brine regenerant makes it difficult to sustain proper osmotic pressures needed to sustain the microorganisms required for denitrification.
Various chemical reduction methods are also used in denitrification. One such oxidation-reduction method is disclosed in U.S. Pat. No. 5,069,880 to Murphy (one of the inventors of this application) and is also described in an article published by Murphy, in Nature 350, 223-225 (1991). Another chemical reduction method is based on immobilized enzymes that proceed via nitrate production and is disclosed in an article by R. B. Mellor et al in Nature, 355, 717-719 (1992). A third chemical reduction method is based on hydrogen and various supported platinum metal catalysts which first produce nitrite, and then finally nitrogen gas or ammonia. (See Platinum Metals Rev. 37, 4, (1993). The disadvantages associated with any of these chemical reduction methods stem primarily from the expense associated with the reducing agent, and the undesirable reaction products left in the effluent which then requires additional post-treatment steps for removal. These are not practical processes for high concentrations of nitrate.
Another water denitrification process uses the precipitation method. However, the precipitation method is not practical for high concentrations (50-500 mg/L as NO.sub.3 --N) because this technology is designed to assist a selected ion exchange denitrification method. The precipitation method does become practical again at very high percentage level of nitrate where no ion exchange technology is needed. The costs associated with the chilling of waste regenerant streams is considered another disadvantage associated with the precipitation method.
Membrane processes are also used for denitrification of water. Nitrate, as an ion in water, can be removed by the use of reverse osmosis membranes along with the other ions. However, there are undesirable costs associated with the overall desalting of the water, in addition to the costs of the reverse osmosis equipment membranes themselves. Furthermore, the increased nitrate present in the reject stream may require further treatment.